The present invention relates generally to photodiode detectors for radiation detection applications. It finds particular application in conjunction with silicone carbide detectors and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other like applications.
Present detector systems for computed tomography (CT) use silicon (Si) diodes in combination with a scintillator plate. The scintillator converts x-ray to visible light photons whereupon a Si photodiode converts these photons to photocurrent. There are numerous problems associated with the use of Si photodiodes. Such problems will become limiting factors for future CT machines.
For example, Si diodes are prone to radiation damage. Radiation damage increases dark current, which decreases internal impedance. Decreased internal impedance results in gain changes, increases in noise level, and makes the conversion of photons to hole-electron pairs less efficient.
Future CT will utilize smaller pixels (individual diode areas), which will produce smaller signal levels. Small signal levels mean that the difference between dark current and signal levels will be much smaller. The dark current level produces an offset. Because the signal level will only be a small factor above the dark current, the dynamic range of the system will be compromised.
Present CT machines are working on the edge of thermal drift problems due to changes in the dark current of Si diodes. The offsets need to be constantly monitored before every scan and corrections are then made to the data for anticipated drift during the scan. This problem will be more severe whenever, as planned, a more powerful x-ray tube is utilized.
More rapid scans will be used in advanced CT machines, which will require faster response scintillators. Present scintillators, which have light wavelength output matched to Si, are slow to respond and contain a light output xe2x80x9ctailxe2x80x9d that decays slowly. Back calculations are required to correct for the slow decay. Consequently, it follows that newer, faster CT machines will require such back calculations to be performed at even faster rates. In fact, the rates at which the back calculations would need to be performed are impractical and, possibly, unachievable.
Oil exploration drilling, for example, uses photo multiplier tubes (PMTs) to detect gamma rays and neutron radiation. This application is similar to the CT application described above in that the detector utilizes a scintillator crystal to convert the gamma and neutron radiation into visible light whereupon the PMT converts this light signal into an electric signal.
The PMTs are extremely expensive and, furthermore, not very reliable (e.g., the failure rate of the PMTs at the bottom of a well near a drill bit is very high). The supply of good PMTs is limited and sorting through hundreds may be required to find a few good ones. Another problem is that the noise of the PMT increases with temperature. Deep hole drilling experiences temperatures as high or higher than 150xc2x0 C. and the increase in PMT noise makes it more difficult to detect radiation. The low reliability of PMTs significantly contributes to the cost of oil exploration drilling. Therefore, more reliable detectors are desirable.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.
In accordance with one aspect of the invention, a radiation detector is provided. It includes: a scintillator which produces UV photons in response to receiving radiation from a radiation producing source; and, a wide bandgap semiconductor device sensitive to the UV photons produced by the scintillator. The semiconductor device produces an electric signal as a function of the amount of UV photons incident thereon.
In accordance with another aspect of the invention, the wide bandgap semiconductor device is a SiC, GaN or AlGaN device.
In accordance with a more limited aspect of the invention, the semiconductor device is a photodiode, an avalanche photodiode or an array of the same.
In accordance with another aspect of the invention, the wide bandgap semiconductor device has a dark current of less than or equal to about 1.0 pA/cm2 at about 0.5 VR.
In accordance with yet another aspect of the invention, the wide bandgap semiconductor device has a bandgap greater than or equal to about 2 eV. Preferably, the bandgap is about 3 eV.
In accordance with another aspect of the invention, an output of the UV photons from the scintillator substantially matches a responsivity of the wide bandgap semiconductor device.
In accordance with still another aspect of the invention, the scintillator is any one of a number of UV scintillators such as those including Li2HfO3, BaF2, Csl, CeF3, LuAlO3:Ce3+, or Lu3Al5O12:Pr3+.
In accordance with another aspect of the invention, the radiation detected is gamma rays or x-rays. One advantage of the present invention is that it uses a photodiode technology having a relatively large yield (e.g., about 50% to 80%).
Another advantage of the present invention is that it incorporates a photodiode having a relatively wide bandgap.
Another advantage of the present invention is that it produces a relatively low dark current.
Another advantage of the present invention is that, because of the wide bandgap and low dark current, it reduces and/or eliminates detector thermal drift problems and detector noise.
Another advantage of the present invention is that SiC photodiodes are more radiation hard than Si photodiodes.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.